These projects are directed towards a greater understanding of the quaternary organization of plasma lipoproteins and of the function of the oligomeric species involved in the transport and metabolism of lipids in plasma. The apolipoprotein composition of plasma lipoproteins is viewed as the governing factor in directing lipoprotein metabolism. Specificity is believed to be related directly to apolipoprotein secondary, tertiary, and quaternary structure. A knowledge of the equilibrium constants and stoichiometry for the specific complexes formed in plasma by apolipoproteins has allowed us to develop a framework for evaluating the role of apolipoproteins in controlling lipid metabolism. These studies have been extended recently to include lipoprotein lipase and hepatic lipase, two enzymes responsible for triglyceride hydrolysis. We have shown that both active and inactive forms of lipoprotein lipase exist in solution. The active form is the dimer and dissociation results in irreversible inactivation. In contrast, the active form of hepatic lipase, which is much more stable in solution than lipoprotein lipase, is the monomer. Differences in the physical properties and stability of these two enzymes may relate to their role in lipid metabolism in vivo.